Conversion of data signals between different signal formats and signal rates can be a challenge, particularly where signal formats use different transport containers and/or independent reference clocks.
The Optical Transport Hierarchy (OTH), for example, is a transport technology for the Optical Transport Network (OTN) developed by the International Telecommunication Union (ITU). The main implementation of OTH is described in two recommendations by the Telecommunication Standardization section of the ITU (ITU-T), including:
Recommendation G.709/Y.1331, entitled “Interfaces for the Optical Transport Network (OTN)”, December 2009, with an Erratum 1 (May 2010), an Amendment 1 (July 2010), and a Corrigendum 1 (July 2010); and
Recommendation G.872, entitled “Architecture of optical transport networks”, November 2001, with an Amendment 1 (December 2003), a Correction 1 (January 2005), and an Amendment 2 (July 2010).
G.709 defines a number of layers in an OTN hierarchy. Client signals are encapsulated into Optical channel Payload Unit (OPUk) signals at one of k levels using either an asynchronous mapping procedure or a synchronous mapping procedure. In the synchronous mapping procedure, the OPUk reference clock is derived from the client signals. This represents the only level of synchronization of transport containers or frames in OTN. Converting between different levels (k) within OTN involves asynchronous processes.